Questions answered

QUESTION:
I try to drink eight glasses of water a day, but instead of drinking one glass every hour or so, I drink three 8-ounce glasses when I wake up in the morning, another three at lunch, then two in the afternoon. Am I getting the necessary hydration that my body needs?

— Debbie Seracini

ANSWER:
Aliens studying earthlings over the past couple decades would almost certainly have noted a strange phenomenon: We have become as attached to our water bottles as Charlie Brown's friend Linus is attached to his blanket. (OK, I confess. A large water bottle is within reach as I write this.)

But is there really any science supporting the idea that it's good and necessary to drink at least eight glasses of water a day? Heinz Valtin, a physician and kidney specialist at Dartmouth Medical School, asked that question in a paper published in the in 2002.

Although Valtin found some evidence that individuals with very low fluid intake are at greater risk for bladder cancer, colorectal cancer, heart disease and migraines, the research studies he examined failed to definitively prove the connection. Overall, Valtin concluded that the wide range of claims for the health benefits of drinking eight glasses of water per day is largely unsupported. He also argued that eight glasses is more water than sedentary individuals in a temperate climate typically need.

In terms of getting the maximum hydration from the water consumed, one study found that a few glasses of water ingested over a couple of hours is largely retained, while the same amount of water ingested in 15 minutes is not.

Individual variation is large, and depends on daily salt intake. The sodium ion concentration in the blood influences a brain sensor called an osmostat, which sends signals that control thirst and water retention.

Water ingested with food is better retained, and contrary to popular belief, it does not slow digestion. Rats allowed to drink during meals digested their food at the same rate as rats deprived of water during their meal.

Water intake can influence calorie intake. On a 12-week diet, middle-aged and older men and women who drank two glasses of water a half hour before meals ate less and lost 5 pounds more than those who did not drink water before meals. Water does not seem to curb appetite in younger adults, but in another study, increasing the water content of the foods themselves decreased calorie intake.